DEFAULT_MESH_DRAIN_BOUND

How long a deduplicated loser is drained before the seam gives up on the remote's goodbye (#2474).

A zombie-link backstop, not the mechanism: a healthy drain terminates in-band on the goodbye, milliseconds after the swap, with no timer involved. Two seconds because the drain holds two things open while it runs — a connection, and the peer's receiver ordering hold — and frame-scale transport hiccups are milliseconds, so this never truncates a real drain.